The present invention relates generally to low voltage CMOS rectifiers. More particularly, the present invention is directed towards a CMOS rectifier with controlled transient response for use in audio amplifier compression and automatic sound level control circuits.
It is desirable in a variety of amplifier applications to have an automatic gain control element that adjusts the gain as a function of input signal level strength. An automatic gain control element may be used by itself or with additional compressor elements to reduce the gain as a function of input signal level strength.
The designers of audio systems describe an xe2x80x9cattackxe2x80x9d time required for an automatic gain control element to adjust the gain of an audio amplifier in response to a sudden increase in audio signal level and a xe2x80x9creleasexe2x80x9d time for the gain to recover after the audio signal level decreases to a normal or average level. Typically, the attack time is defined as the time that it takes for an audio amplifier to stabilize to within about 1 dB of its final value after a sudden increase in input audio level. The release time is typically defined as the time that it takes for an audio amplifier to decrease to within about 1 dB of its final value after the input audio level has decreased to normal levels. Typically, it is desirable that an audio amplifier have a release time that is much longer than its attack time.
A short attack time is desirable to produce a comfortable reduction in sound level after an abrupt increase in sound level, e.g., the ringing of an alarm bell.
However, an abrupt return to ordinary gain levels when the audio signal level decreases to ordinary levels may sound unnatural to many users. Consequently, it is typically desirable that the release time be much longer than the attack time. As an illustrative example, a preferred attack time for an automatic gain control used in an audio compressor may be less than one millisecond whereas a preferred release time may be ten milliseconds.
More generally, the transient response of an automatic gain control element can also be described in terms of an attack function and a release function. Commonly, the attack and release function is in the form of an exponential response, since a variety of passive circuits comprised of resistors and capacitors may be used to implement an exponential response. For a simple exponential response, the exponential time constant describes the attack/release function.
An automatic gain control element may be used to perform several different functions. One function of an automatic gain control element is as an automatic sound level controller that maintains an average audio output level, or loudness, that is comfortable to the listener. This has the advantage that the user does not need to manually adjust the sound level to achieve a comfortable level of sound amplification as the background sound level varies. Another function of an automatic gain control element is to compress the gain at high input signal levels in order to avoid the generation of distortion which may otherwise occur at such signal levels. As is well-known in the field of amplifier design, amplifiers typically produce a distorted output when the input signal level exceeds a preferred level. For example, when the inputs to an amplifier become too large the output of a xe2x80x9clinearxe2x80x9d amplifier may become saturated at a constant power-output regardless of further increases in input signal level strength, resulting in non-linear (distorted) amplification. Still another function of an automatic gain control element is as a syllabic compressor. A syllabic compressor automatically compresses sound as a function of amplitude according to a function that is designed to improve the ability of the listener to understand certain sounds, such as distinguishing between consonant and syllabic sounds.
Although a variety of compressors are known in the amplifier art, compressors suitable for miniature hearing aids pose special problems. First, there are severe space limitations in miniature hearing aids. Hearing aid designers are forced by space constraints to severely limit the number of discrete capacitors and resistors in the total hearing aid circuit, since discrete components greatly exacerbate the problems of fitting all of the hearing aid components into a unit sized to fit partially or totally in the ear canal. Thus, an audio compressor for use in a miniature hearing aid should utilize a minimum number of discrete capacitors and resistors. Second, there are severe power supply limitations. Miniature hearing aids are powered by a single miniature hearing aid battery with a nominal voltage of less than 1.5 volts. Consequently, an audio compressor circuit for use in a hearing aid should be consistent with low voltage operation. Third, cost is an important consideration for the low-cost hearing aid market. A hearing aid integrated circuit, including one or more compressors, should be compact in order to reduce the per-unit cost.
Hearing aids are one of the most common examples of miniature audio devices in which it is desirable to incorporate automatic gain control elements, typically as audio compressors. However, a wide variety of other miniature audio devices, such as cellular phones and micro-recorders, may also beneficially use automatic gain control elements as syllabic compressors, gain compressors, or as a sound level controllers to improve the sound quality of transmitted or recorded speech. There is a general need for a miniature control circuit that may be used in combination with a voltage-controlled amplifier as part of a compressor with selectable attack/rise characteristics.
Conventional audio compressors used in hearing aids commonly utilize signal characterization elements requiring at least two discrete capacitors and one discrete resistor to implement an attack/release function in a compressor. For example, U.S. Pat. No. 4,718,099 discloses a voltage controlled compression amplifier with controlled attack and release time constants designed for use as a compressor in a hearing aid. As shown in the prior art block circuit diagram of FIG. 1, a voltage controlled amplifier 10 has an output 12 that may be fed into other amplifier elements 26, 28 that are suitably coupled by connections 27, 29 to a hearing aid receiver. The gain of voltage controlled amplifier 10 is further regulated by a gain control terminal 13 coupled 15 to a gain control voltage source 14. Gain control voltage source 14 includes a Shotcky diode (not shown in FIG. 1) to produce a rectified current 16 when the output 12 exceeds a predetermined threshold level. The rectified current 16 of gain control voltage source 14 is coupled to node 24 of a signal characterizer 20. The function of signal characterizer 20 is to convert an input signal 16 into a form suitable for regulating the voltage at gain control terminal 13, i.e., to decrease the gain of amplifier 10 with an appropriate attack time in response to an abrupt increase in amplifier output 12 and to restore the gain of amplifier 10 with an appropriate release time when the amplifier output 12 decreases to normal levels. Signal characterizer 20 comprises a first capacitor 21 in parallel with a series connection of resistor 23 and second capacitor 22. The signal characterizer 20 has a short attack time corresponding to the equivalent RC turn on time of first capacitor 24 and a a slow release time corresponding to the equivalent RC turn-off time of a second capacitor 22 and its associated resistor 23.
One drawback of the automatic gain control circuit of U.S. Pat. No. 4,718,099 is that signal characterizer 20 requires comparatively large discrete resistors and discrete capacitors in order to achieve RC time constants of capacitors 21, 22 consistent with attack and release times on the order of microseconds. For example, in a preferred embodiment of U.S. Pat. No. 4,718,099, a first time constant means comprises a 0.47 microfarad capacitor and a second time constant means comprises a second capacitor with a capacitance of 2.2 microfarads and a resistor with a resistance of 220 kilo-ohms. As is well known in the art of CMOS design, it is impractical to fabricate capacitors with extremely large capacitance values in an integrated circuit. Consequently, the preferred embodiment of U.S. Pat. No. 4,718,099 requires two external capacitors for its implementation. This is highly undesirable for a miniature hearing aid, since it is difficult in modem miniature hearing aids to incorporate two external capacitors into the compact geometry of in-the-ear and in-the-canal hearing aids. Moreover, while large value resistors (e.g., greater than 100 kilo-ohms) may be implemented in modern CMOS processes, large value resistors typically consume a large amount of chip area and are subject to lot-to-lot fabrication variations. Thus, U.S. Pat. No. 4,718,099 may also require a large value external resistor for its implementation in a high yield fabrication process, further increasing the difficulty of manufacturing a miniature hearing aid incorporating signal characterizer 20.
Another drawback of the automatic gain control circuit of U.S. Pat. No. 4,718,099 is that the attack/release function is limited to having simple exponential rise/fall characteristics. The signal characterization function that capacitors 21, 22 and other resistors may perform is intrinsically limited by well known mathematically relationships describing an exponential rise/decay of the voltage of capacitors 21, 22 in response to abrupt changes in control signal 16. While a compression function based upon a simple exponential rise/fall transient response may be desirable for some audio background environments, it is known in the art of hearing aid design that a simple exponential response does not produce an acceptable response for all hearing aid users in all audio environments. In particular, many hearing aids are returned because the compression circuits produce a so-called xe2x80x9cpumpingxe2x80x9d effect in which the compression function strongly depends upon the background noise levels. Capacitors 21, 22 of signal characterizer 20 receive a rectified current from Shotcky diode (not shown) when the output is above a threshold signal level. In a noisy background environment, particularly one punctuated by repetitive or frequent loud noises (e.g., traffic noises on a busy street), capacitors 21, 22 will be charged up by the background noises. Consequently, the compression response of capacitors 21, 22 to a single audio pulse in a quiet room will be fundamentally different than that which occurs in a noisy background environment, particularly an environment punctuated by periods of repetitive high noise levels (e.g., the sound of traffic noises on a busy street). In some environments, the background noises will xe2x80x9cpumpxe2x80x9d the compression up and down in a manner that results in the gain of desired sounds rising and falling in an unpleasant and unnatural sounding manner. For example, a noisy background environment may result in pumping that causes the sound of normal speech to fade in and out in an unpleasant manner.
A general problem in a variety of miniature audio devices is that it is difficult to implement a signal characterization circuit that provides a voltage to a voltage controlled amplifier with a controlled attack and release time. This problem is particularly acute in the context of miniature hearing aids because of the voltage and space constraints.
What is desired is a new design for a control element that provides a rectified control voltage with a selectable transient response function.
The present invention is a rectifier that provides an output that is a function of the magnitude of the input signal but with a selectable attack/release response. The present invention generally comprises: a capacitor; a charging current element coupled to the capacitor having a charging switch to regulate the flow of a charging current into the capacitor; a discharge current element coupled to said capacitor having a discharge switch to regulate the flow of a discharging current out of the capacitor; a switch controller with outputs for alternately turning on the charging switch and the discharge switch, the switch controller having a first signal input and a second input corresponding to the voltage on the capacitor; electrical connections for coupling the outputs of said switch controller to the switches; and electrical connections for coupling the capacitor voltage and an input signal to the switch controller; wherein the charging switch provides control signals to said switches when the magnitude of the input signal is greater than the capacitor voltage and the discharge switch is turned on when the magnitude of the input signal is less than the capacitor voltage.
In a preferred embodiment, the charging current elements and discharging current element comprise switched constant current sources. The switched constant current sources are preferably current mirrors with charging/discharging switches arranged to turn on/off the mirror currents of the current mirrors.
One object of the present invention is a rectifier that is extremely sensitive to very small voltage changes, such as voltage changes of about 250 millivolts.
Another object of the present invention is a low-voltage rectifier requiring a reduced number of discrete capacitors and resistors for its implementation.
Still another object of the present invention is a rectifier with a selectable transient response. In the preferred embodiment, the rectifier output voltage increases/decreases linearly with time in response to abrupt changes in input signal level magnitude.
Yet still another object of the present invention is the ability to sum the outputs of a plurality of rectifiers together to permit the transient response to be further adjusted.